1. Field of the Invention
This document relates to a method and apparatus of determining arrival time of data packet when a digital broadcast data stream is recorded.
2. Description of the Related Art
Optical disks have been popular media to record video contents instead of magnetic tapes. A representative one of them is DVD+/−R that accommodates video contents of about 1 to 6 hours depending on writing modes.
At present, TV broadcast signals have been provided in the digital form and the digital TV broadcast signals provide more higher quality than analog TV signals. Hence a new disk medium whose recording volume is much larger than that of DVD+/−R is required to record the digital TV broadcast signals and development of those kinds of optical disks has been on progress.
An example of those kinds of disks is HD-DVD. FIG. 1 illustrates that a digital broadcast stream is recorded on a HD-DVD. To be specific, MPEG transport packets, each of which has a fixed length of 188-bytes (hereinafter referred to as TP) are recorded in the order that they have been arrived. For each TP, packet arrival time stamp (PATS) indicative of arrival time of the TP is added in the front of the TP, as shown in FIG. 1.
On the other hand, as shown in FIG. 2A, a program clock reference (PCR) is disposed in the TP header for every at least 100 milliseconds. The PCR value is encoded into two fields called PCR—base of 31 bits and PCR_ext of 9 bits. The PCR_ext has a modulo of 300 and is a value counted by a clock 27 Mhz while the PCR_base is a value counted by carries from the PCR_ext. Because the carry is produced from the PCR_ext every 300 counts, the PCR_base is counted by 90 kHz(=27 MHz/300) clock.
An apparatus of recording the TPs adjusts its system clock based on the PCRs within a TP sequence. Specifically, the system clock is adjusted in such a way that given two consecutive PCRs within a TP stream, the difference between receiving times of the two PCRs, which are represented by counting value by the system clock, becomes equal to the difference between two consecutive PCR values. Therefore, it is preferred that the system clock in the recoding apparatus operates at 27 Mhz which is equal to the frequency at which PCRs are generated.
For some reasons, some recording apparatuses have different system clock frequency from 27 MHz. For example, by using a personal computer equipped with a digital TV broadcast signal receiver, a user wishes to record the digital TV signals on a hard disk. In general, personal computers do not have the clock frequency of 27 MHz.